MD5 for Large Files

[Richard Lemay]

I am new to VB.NET and I am trying to learn. So, your indulgence for the
triviality of my questions is kindly requested.

I would like to calculate an MD5 hash for very lage files. The examples I
came across read the file into a byte array and apply the hash to that
array. The following code illustrates what I am doing. I would like to
perform the hash calculation on a stream. Is this possible? If so, an
example would be greatly appreciated.

Imports System.Text
Imports System.Security .Cryptography
Imports System.IO

Public Class Form1
Inherits System.Windows. Forms.Form
Dim fs As FileStream = New FileStream("c:\ ConfDenise.PDF" ,
FileMode.Open)
Dim r As BinaryReader = New BinaryReader(fs )

Public Function GenerateHash(By Ref Buff() As Byte) As String
Dim Md5 As New MD5CryptoServic eProvider()
Dim ByteHash() As Byte = Md5.ComputeHash (Buff)
Return Convert.ToBase6 4String(ByteHas h)
End Function

#Region " Windows Form Designer generated code "
' snip
#End Region

Private Sub Button1_Click(B yVal sender As System.Object, ByVal e As
System.EventArg s) Handles Button1.Click
fs.Seek(0, SeekOrigin.Begi n)
Label1.Text = GenerateHash(r. ReadBytes(fs.Le ngth))
End Sub
End Class

--
Richard
Please reply to group or
Remove 'REMOVE' from my email address.

[Rob Teixeira [MVP]]

If you are familiar with Block cipher encryption in .NET, hashes are
supposed to work the same way.
In other words, hash algorithms (like MD5) implement the ICryptoTransfor m
interface, which allows them to be used by the CryptoStream object.
You could create a memory stream, and wrap it with a CryptoStream, and pass
the MD5 instance to the CryptoStream constructor. Anytime you "write" to the
CryptoStream, it should transform the data through the protecte HashCore
function of MD5. Then, when you are done, you call FlushFinalBlock on the
CryptoStream, which in turn calls the MD5 HashFinal method. Then you can
check the Hash property of the md5 object for the final hash value.

Function GetHash() As String

Dim cs As CryptoStream
Dim ms As MemoryStream = New MemoryStream
Dim md5Hash As MD5CryptoServic eProvider = New MD5CryptoServic eProvider

Try

Dim buffer As Byte() = New Byte() {1, 2, 3, 4, 5, 6, 7, 8, 9, 10,
11, 12, 13, 14, 15}

cs = New CryptoStream(ms , md5Hash, CryptoStreamMod e.Write)
cs.Write(buffer , 0, buffer.Length)

cs.FlushFinalBl ock()
Return Convert.ToBase6 4String(md5Hash .Hash())

Catch ex As Exception

MsgBox("Error during hash operation: " + ex.ToString())

Finally
If Not (cs Is Nothing) Then cs.Close()
If Not (md5Hash Is Nothing) Then md5Hash.Clear()
End Try
End Function

The only thing to note is that in my sample, i just filled the buffer once
with a bunch of numbers.
In your case, you need to wrap a Loop around the cs.Write(...) line, and
read from your file to the buffer one chunk at a time, and write the buffer
to the CryptoStream until you run out of file.

-Rob Teixeira [MVP]

"Richard Lemay" <rl************ *********@mailb locks.com> wrote in message
news:ul******** ******@TK2MSFTN GP11.phx.gbl...

I am new to VB.NET and I am trying to learn. So, your indulgence for the
triviality of my questions is kindly requested.

I would like to calculate an MD5 hash for very lage files. The examples I
came across read the file into a byte array and apply the hash to that
array. The following code illustrates what I am doing. I would like to
perform the hash calculation on a stream. Is this possible? If so, an
example would be greatly appreciated.

Imports System.Text
Imports System.Security .Cryptography
Imports System.IO

Public Class Form1
Inherits System.Windows. Forms.Form
Dim fs As FileStream = New FileStream("c:\ ConfDenise.PDF" ,
FileMode.Open)
Dim r As BinaryReader = New BinaryReader(fs )

Public Function GenerateHash(By Ref Buff() As Byte) As String
Dim Md5 As New MD5CryptoServic eProvider()
Dim ByteHash() As Byte = Md5.ComputeHash (Buff)
Return Convert.ToBase6 4String(ByteHas h)
End Function

#Region " Windows Form Designer generated code "
' snip
#End Region

Private Sub Button1_Click(B yVal sender As System.Object, ByVal e As
System.EventArg s) Handles Button1.Click
fs.Seek(0, SeekOrigin.Begi n)
Label1.Text = GenerateHash(r. ReadBytes(fs.Le ngth))
End Sub
End Class

--
Richard
Please reply to group or
Remove 'REMOVE' from my email address.

[Richard Lemay]

Thanks Rob,
I'll give it a try.
Richard

"Rob Teixeira [MVP]" <RobTeixeira@@m sn.com> wrote in message
news:%2******** ********@tk2msf tngp13.phx.gbl. ..

If you are familiar with Block cipher encryption in .NET, hashes are
supposed to work the same way.
In other words, hash algorithms (like MD5) implement the ICryptoTransfor m
interface, which allows them to be used by the CryptoStream object.
You could create a memory stream, and wrap it with a CryptoStream, and pass the MD5 instance to the CryptoStream constructor. Anytime you "write" to the CryptoStream, it should transform the data through the protecte HashCore
function of MD5. Then, when you are done, you call FlushFinalBlock on the
CryptoStream, which in turn calls the MD5 HashFinal method. Then you can
check the Hash property of the md5 object for the final hash value.

Function GetHash() As String

Dim cs As CryptoStream
Dim ms As MemoryStream = New MemoryStream
Dim md5Hash As MD5CryptoServic eProvider = New MD5CryptoServic eProvider

Try

Dim buffer As Byte() = New Byte() {1, 2, 3, 4, 5, 6, 7, 8, 9, 10,
11, 12, 13, 14, 15}

cs = New CryptoStream(ms , md5Hash, CryptoStreamMod e.Write)
cs.Write(buffer , 0, buffer.Length)

cs.FlushFinalBl ock()
Return Convert.ToBase6 4String(md5Hash .Hash())

Catch ex As Exception

MsgBox("Error during hash operation: " + ex.ToString())

Finally
If Not (cs Is Nothing) Then cs.Close()
If Not (md5Hash Is Nothing) Then md5Hash.Clear()
End Try
End Function

The only thing to note is that in my sample, i just filled the buffer once
with a bunch of numbers.
In your case, you need to wrap a Loop around the cs.Write(...) line, and
read from your file to the buffer one chunk at a time, and write the buffer to the CryptoStream until you run out of file.

-Rob Teixeira [MVP]

"Richard Lemay" <rl************ *********@mailb locks.com> wrote in message
news:ul******** ******@TK2MSFTN GP11.phx.gbl...

I am new to VB.NET and I am trying to learn. So, your indulgence for the triviality of my questions is kindly requested.

I would like to calculate an MD5 hash for very lage files. The examples I came across read the file into a byte array and apply the hash to that
array. The following code illustrates what I am doing. I would like to
perform the hash calculation on a stream. Is this possible? If so, an
example would be greatly appreciated.

Imports System.Text
Imports System.Security .Cryptography
Imports System.IO

Public Class Form1
Inherits System.Windows. Forms.Form
Dim fs As FileStream = New FileStream("c:\ ConfDenise.PDF" ,
FileMode.Open)
Dim r As BinaryReader = New BinaryReader(fs )

Public Function GenerateHash(By Ref Buff() As Byte) As String
Dim Md5 As New MD5CryptoServic eProvider()
Dim ByteHash() As Byte = Md5.ComputeHash (Buff)
Return Convert.ToBase6 4String(ByteHas h)
End Function

#Region " Windows Form Designer generated code "
' snip
#End Region

Private Sub Button1_Click(B yVal sender As System.Object, ByVal e As
System.EventArg s) Handles Button1.Click
fs.Seek(0, SeekOrigin.Begi n)
Label1.Text = GenerateHash(r. ReadBytes(fs.Le ngth))
End Sub
End Class

--
Richard
Please reply to group or
Remove 'REMOVE' from my email address.